Strokes happen when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. In both cases, brain cells start dying within minutes. Nearly nine out of ten strokes are caused by a blockage, while the rest result from bleeding. About 90% of the overall stroke burden traces back to risk factors people can change, which means understanding how strokes happen is the first step toward preventing one.
Blocked Blood Flow: Ischemic Stroke
The most common type of stroke occurs when something blocks an artery feeding the brain. Without a steady supply of oxygen and nutrients, brain tissue downstream of the blockage begins to die. This triggers a chain reaction of damage: cells release toxic levels of signaling chemicals, inflammation ramps up, and oxidative stress compounds the injury.
Blockages happen in two main ways. In a thrombotic stroke, a blood clot forms directly inside one of the arteries supplying the brain. This usually happens at a spot where fatty plaque has already narrowed the vessel. In an embolic stroke, the clot forms somewhere else in the body, breaks free, and travels through the bloodstream until it gets wedged in a smaller artery in the brain. The end result is the same: part of the brain loses its blood supply.
The speed of damage is staggering. For every minute a large-vessel stroke goes untreated, the average patient loses 1.9 million neurons, 14 billion synapses, and 7.5 miles of nerve fibers. That’s why emergency treatment focuses on restoring blood flow as fast as possible.
Where Clots Come From
The Carotid Arteries
The carotid arteries run up each side of the neck and are the brain’s main blood supply. Over time, cholesterol, fat, and blood cells can build up inside these arteries as plaques, a process called atherosclerosis. As plaque accumulates, the artery narrows, reducing blood flow. But the bigger danger is when a piece of plaque breaks off. That fragment travels into smaller arteries in the brain and gets stuck, cutting off blood supply to the tissue beyond it.
The Heart
A heart rhythm disorder called atrial fibrillation (AFib) is one of the most significant sources of stroke-causing clots. During AFib, the upper chambers of the heart quiver instead of contracting in a coordinated way. Blood pools and stagnates, especially in a small pouch called the left atrial appendage. Stagnant blood clots easily. If one of those clots gets pumped out of the heart and reaches the brain, it causes an embolic stroke. AFib also tends to coexist with other structural changes in the heart, including scarring, chamber enlargement, and dysfunction of the blood vessel lining, all of which further increase clotting risk.
Bleeding in the Brain: Hemorrhagic Stroke
Hemorrhagic strokes account for roughly one in ten cases, but they tend to be more severe. They occur when a blood vessel in or around the brain ruptures, and blood pools in surrounding tissue. That pooling blood puts direct pressure on brain cells and disrupts normal function.
The most common cause is a burst brain aneurysm, a weak spot in an artery wall that balloons outward over time. When it ruptures, blood spills into the space between the brain and its protective covering. This is called a subarachnoid hemorrhage. Another cause is an arteriovenous malformation, an abnormal tangle of blood vessels that can rupture and bleed directly into brain tissue. High blood pressure, which weakens vessel walls over years, is a major contributor to both types.
How High Blood Pressure Damages the Brain
Chronic high blood pressure is the single largest modifiable risk factor for stroke, responsible for roughly 65% of strokes in older adults and 29% in younger adults. The damage it causes is gradual and often invisible until a stroke occurs.
Sustained high pressure stiffens the large arteries, particularly the aorta. That stiffness forces stronger pressure pulses deeper into the brain’s tiny blood vessels, which aren’t built to handle that kind of force. Over time, those small vessels remodel and narrow, reducing blood flow and making them more fragile. Research in brain imaging has linked this excessive pulsatility to small areas of tissue damage called lacunar infarcts, along with loss of both white and gray matter volume. The process creates a feedback loop: stiffer arteries raise blood pressure, which stiffens arteries further, progressively increasing the risk of both blockages and ruptures.
Strokes With No Clear Cause
About 25% of ischemic strokes are classified as cryptogenic, meaning doctors can’t identify a definitive cause even after standard testing. In younger patients, a surprisingly common finding is a patent foramen ovale (PFO), a small hole between the upper chambers of the heart that normally closes shortly after birth but persists in some people. A PFO shows up in 40% to 50% of young stroke patients with no other explanation, compared to only 10% to 15% of people without strokes. The hole can allow small clots from the venous system to cross into the arterial side and reach the brain, bypassing the lungs where they would normally be filtered out. Analysis suggests a PFO may be the actual culprit in roughly two-thirds of these unexplained cases, and possibly up to 80% in younger patients.
Transient Ischemic Attack
A transient ischemic attack, often called a mini-stroke, happens when blood flow to the brain is temporarily blocked but restores on its own, usually within minutes. Symptoms mirror a full stroke (sudden numbness, confusion, trouble speaking, vision changes) but resolve quickly. A TIA doesn’t cause permanent brain damage, but it’s a serious warning sign. It means the same mechanism that causes a full stroke, whether a narrowed artery or a traveling clot, is actively at work.
Risk Factors You Can Change
About 90% of the global stroke burden is tied to modifiable risk factors. That statistic, drawn from both the Global Burden of Disease Study and the large international INTERSTROKE study, means most strokes are not inevitable. The specific factors that matter most shift with age.
For younger adults, smoking and high blood pressure are the two biggest contributors, accounting for roughly 34% and 29% of strokes in that age group, respectively. For older adults, high blood pressure dominates at about 65%, with diabetes as the second largest factor at around 26%. Physical inactivity, poor diet, and obesity round out the major modifiable risks across all age groups.
One useful way to think about it: five common risk factors (high blood pressure, smoking, diabetes, physical inactivity, and obesity) collectively explain about 80% of ischemic stroke cases. Addressing even one of them meaningfully lowers your odds, and addressing several compounds the benefit.